MAINTAINERS: Add co-maintainer for Renesas Pin Controllers

Add myself as a co-maintainer for Renesas Pin Controllers.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Linus Walleij <linus.walleij@linaro.org>